Field of the Invention
This invention relates to glasses having a low or high oxidized iron content, to methods of making such glasses, and to articles made using such glasses, and more particularly, the invention relates to a lithium aluminosilicate glass having a high or low infrared transmission, a method of changing from a Campaign making high infrared absorbing glasses, i.e., a glass having high reduced iron content, to a Campaign making a low infrared absorbing glass, i.e., a glass having low reduced iron content, and vice versa, and articles, e.g. automotive and/or aircraft transparencies made using such glasses.
Discussion of the Technology
Of particular interest in the following discussion is the manufacture of lithium containing glasses. As is appreciated by those skilled in the art, ion exchanged strengthened glass is frequently made using lithium containing glasses. One type of lithium containing glass is disclosed in U.S. Pat. No. 4,156,755 (“hereinafter also referred to as “USPN '755”), which patent in its entirety is incorporated herein by reference. The glass of USPN '755 is also referred to as a “lithium aluminosilicate glass” as that reference covers the three components of the glass, namely lithium, aluminum and silica that are most characteristic to the glass.
In general, iron is not a required ingredient to make lithium aluminosilicate glass for the ion exchange process, however, small amounts of iron are usually present in the lithium aluminosilicate glass as an impurity in the glass batch ingredients. At the present time, the management of iron oxide in the lithium containing glass batch materials to alter the optical and/or color properties of the lithium containing glass is of present interest. By way of background interest, total iron oxide content as Fe2O3 in commercial glasses depends on the product requirements, but are commonly in the range of 50-1200 parts per million (hereinafter also referred to as “PPM”) or 0.005-0.12% of the total by weight (hereinafter referred to as “percent by weight” or “wt. %”) for what are considered clear glass compositions. More particularly, the addition of iron to the glass composition can be made as ferrous iron (FeO) or as ferric iron (Fe2O3). During the melting of the glass batch materials, equilibrium is reached between the ferric form of iron (Fe+++) and the ferrous form of iron (Fe++) with about 25-30 wt. % of the iron in the ferrous form (Fe++) and 70-75 wt. % of the iron in the ferric form (Fe+++). The ferric oxide, Fe2O3, is a strong ultraviolet radiation absorber and operates as a yellow colorant in the glass, and the ferrous oxide, FeO, is a strong infrared radiation absorber and operates as a blue colorant in the glass.
In the instance when a glass sheet, for example but not limiting to the discussion, a lithium aluminosilicate glass sheet (hereinafter also referred to as a “lithium glass sheet”) is to be heated, e.g. but not limiting to the discussion, prior to bending and/or shaping, the composition of the lithium glass sheet preferably includes ferrous oxide (FeO) in the range of 0.02 to 0.04 wt. % (200 to 400 PPM), ferric oxide (Fe2O3) in the range of 0.05 to 0.10 wt. % (500 to 1000 PPM), and a redox ratio (discussed in detail below) in the range of 0.2 to 0.4. In the instance when a lithium glass sheet is to be used in the practice of the invention as a viewing window for infrared equipment, e.g. but not limited to, infrared night goggles, or as components of transparent armor or aerospace windows, the composition of the lithium glass sheet preferably includes ferrous oxide (FeO) preferably in the range of 0.001 to 0.005 wt. % (10-50 PPM), ferric oxide (Fe2O3) in the range of 0.010 to 0.05 wt. % (100-500 PPM), and the lithium glass has a preferred redox ratio in the range of 0.005 to 0.10. As noted from the above discussion, the wt. % of ferrous oxide is higher and the ferric oxide is lower for the lithium glass sheet to be heated to increase the absorption of the infrared wavelengths to decrease the heating time of the lithium glass sheet to reach the bending temperatures, and the wt. % of the ferrous oxide is lower and the ferric oxide is higher for the lithium glass sheet to be used for a viewing window for infrared equipment to increase the percent transmittance of infrared energy through the viewing window.
Consider now the drawbacks of going from a Campaign making a high infrared absorbing (hereinafter also referred to as “HIRA”) lithium glass sheet to a Campaign making low infrared absorbing (hereinafter also referred to as “LIRA”) lithium glass sheet, and/or going from a Campaign making a LIRA lithium glass sheet to a Campaign making HIRA lithium glass sheet. As can now be appreciated by one skilled in the art one drawback is the quantity of glass produced during the period starting at the end of one Campaign, e.g. the end of the Campaign to make HIRA lithium glass sheet, and ending at the start of the next Campaign, e.g. the start of the Campaign to make LIRA lithium glass sheet. The glass that is out of specifications for use as LIRA lithium glass sheet and HIRA lithium glass sheet is usually scrapped or used as cullet. It can now be appreciated by those skilled in the art that discarding the glass made during the change from one Campaign to another Campaign is costly due to the relatively high batch cost for lithium glass and to the time wasted making unusable glass or glass of marginal quality.
It is advantageous, therefore, to provide a method of minimizing or eliminating the drawbacks associated with changing from a Campaign making useable HIRA lithium glass sheet, or useable LIRA lithium glass sheet to a Campaign making useable LIRA lithium glass sheet, or useable HIRA lithium glass sheet, respectively.